<!DOCTYPE html>
<html>
    <head lang="en">
        <meta charset="UTF-8">
        <title></title>
        <link rel="stylesheet" href="1.css" type="text/css">
    </head>
    <body>
        <div id="div" class="div">
            hello
        </div>
        <button onclick="demo()">按钮</button>
        <script>
        function demo(){
            document.getElementById("div").style.background="blue"
        }
        </script>
    </body>
</html>